1. Field of the Invention
The present invention relates to a vacuum degassing system and more particularly pertains to a method and apparatus associated with removing gases from liquids in a flow-through relation in which an elongated gas-permeable tube addresses a vacuum chamber evacuated by a variable speed vacuum pump and gas is transferred by diffusion through the walls of the tube. The system is particularly suited to the removal of air or oxygen from the mobile phase associated with high performance liquid chromatographic equipment.
2. Description of the Prior Art
There are many chemical applications, particularly analytical applications, involving the use of liquid solvents, reactants or the like in which the presence of dissolved gases, particularly air, is undesirable. A prime example of such an application relates to the mobile phase in high performance liquid chromatography where the presence of even small amounts of dissolved gases, and in particular oxygen, interferes with the accuracy and sensitivity of the results obtained. For example, air dissolved in the mobile phase can manifest itself in the form of bubbles which causes noise and drift as the mobile phase passes through the detector. If the dissolved species be chemically active, as in the case of oxygen in air, it can additionally produce unwanted changes or deterioration in the mobile phase. Of course, the detrimental effect of the dissolved species is related to the relative concentration of the species in the mobile phase. These undesirable species usually are removed by a degassing process. It correspondingly follows that the more efficient the removal or degassing system is, the more desirable it will be.
The degassing of liquid materials has been necessary to the success of many processes and, consequently, the process has been pursued actively in several forms for a long period of time. Techniques have included heating or boiling the liquid to be degassed, exposing the material to a reduced pressure environment or vacuum and using combinations of heat and vacuum to reduce the amount of dissolved gases in the liquid. Ultrasonic energy has also been employed. As conventionally applied, however, these traditional techniques have generally fallen short of the desired degree of separation efficiency. Additionally, a means of degassing solvent involving the passing of a fine stream of bubbles of inert gas such as helium through the solution to be degassed has been shown by Bakalyar et al. in U.S. Pat. No. 4,133,767, and in apparatus such as that disclosed by Sims et al. in U.S. Pat. No. 4,994,180, co-invented by the co-inventor in the present application and assigned to the same assignee as the present invention.
Vacuum degassing through a membrane apparatus has long been known, and generally utilizes a length of relatively small diameter, thin-walled semi-permeable synthetic polymer resin material contained within an enclosed chamber held under a reduced pressure or vacuum in which the liquid to be degassified is caused to flow through the tube. One such apparatus is shown by Sims in U.S. Pat. No. 5,340,384, co-invented by the co-inventor in the present application and assigned to the same assignee as the present invention. Other such devices are shown in U.S. Pat. Nos. 5,183,486, 4,430,098, and 3,668,837.
While each of these devices employ a flow-through tube vacuum degassing approach, there remains a need, particularly with devices associated with high performance liquid chromatography instruments, to make degassing of solvents, and in particular the mobile phase, more efficient. One particular limitation or drawback associated with present devices concerns the efficiency of the degassification operation with respect to the composition of the tubing itself. Materials presently used in degassing applications include PTFE, PFA, and silicone rubber. These materials, while generally suitable for this application, require that the wall thickness be as thin as possible due to the gas permeability of materials typically utilized for these applications. A large internal diameter tube is disadvantageous as the gas must diffuse through a longer path from the center of the flow to the inner wall surface, thereby requiring a long tube. Additionally, a tube of greater length increases flow resistance through the overall system the resistance being a linear function of tubing length (assuming laminar liquid flow through the tubing). Liquid flow resistance is an inverse function of tubing internal diameter to the fourth power.
Amorphous perfluorinated copolymers reportedly have permeabilities of up to 2 or 3 orders of magnitude higher than those of PTFE. It has been found by the present inventors that by using amorphous perfluorinated copolymers, such as those marketed by Du Pont under the tradename Teflon AF that permeabilities of up to about 1 order of magnitude or greater are experienced. Nevertheless, in the fabrication of degassing tubes, greater gas mass transfer rates can be achieved with tubes of Teflon AF having increased wall thicknesses, thereby permitting the undertaking of applications requiring higher pressures. Advantageously, tubes of smaller internal diameter and shorter length offer reduced internal volumes. Low flow resistance is accomplished with multi-lumen tubing arrangements.
Because of the enhanced gas permeability property of materials utilized in accordance with the present invention, the diffusion rate of atmospheric gases from the liquid being degassed through the tubing wall is significantly increased. It appears likely that the increased gas permeability enhances the function of free (void) volume in the polymer component.
As a further feature of this invention, it has been found that very stable reduced pressure or vacuum is achieved within the vacuum chamber. This feature is possible due to the operational characteristics of the vacuum pump. In initial operation, the pump (typically operated @ 400 RPM) reduces pressure inside the vacuum chamber. When the partial pressure inside the chamber begins to asymptotically approach a maximum differential value (typically around 60 mm Hg absolute) the speed is substantially reduced, such as to about 60 RPM. The pump is run continuously at this reduced rate, with the vacuum then slowly descending to a "constant vacuum level" with the pressure remaining constant for so long as the pump is running. This "constant vacuum level" provides significant advantages in that it eliminates vacuum hysteresis (pressure) which typically is in the range of 15-25 mm Hg as the result of cycling the pump on and off in other systems. Through this operational feature, variations in remaining atmospheric gas in the mobile phase exiting the degassing apparatus to the liquid chromatograph are also eliminated. This feature provides technical advantages because of the resulting HPLC detector base line stability. Superior vacuum level, typically in the range of 30 mm Hg or less, also reduces the absolute concentration of dissolved gases in the mobile phase, which improves the flow rate precision of the HPLC pump. In addition, longer life expectancy is available because of low RPM.
Accordingly, it is a principal object of the present invention to provide a more efficient vacuum degassing system of the flow-through type using a tube or multiple tubes formed from an amorphous perfluorinated copolymer.
A further object of the present invention is to reduce the required inside diameter and length of the degassing tube.
A still further object of the present invention is the provision of a tube having a single lumen.
A yet further object of the present invention is the provision of a tube having multiple lumens.
Another object of the present invention is the provision of a variable speed pump and run continuously for evacuating the vacuum chamber, with the effects being a reduction or elimination of hysteresis and increased vacuum pump life expectancy.
Still further object of the present invention is the utilization of a flow restrictor in the system to facilitate bleeding or flushing of the head with atmosphere to prevent solvent vapor build up.
Still another object of the present invention is to provide a means for interconnecting liquid chromatography instrument components which simultaneously degasses the mobile phase in transit between the components.
It is yet a further object of the present invention to provide an improved vacuum pump head bleed or flush system in the form of a vent frit including a sintered metal plug, thereby eliminating the necessity of the solenoid operated vent valve typically utilized in current systems.
A still further object of the invention is to provide an improved connection system for the tube associated with a flow-through vacuum degassing apparatus.